Electric utilities need reliable sources of energy. Because generation must match electrical load, which varies throughout the day, they want generation resources that can be turned on or off, or up or down anytime, like a water faucet. They call such sources of energy dispatchable.
They consider a hydro-electric plant to be dispatchable because the river is always flowing. They consider a natural gas ‘peaker’ plant to be dispatchable because it can be fired up at any time.
The conventional view of photovoltaic (PV) solar arrays is that they are not dispatchable, because the sun is not always shining on them. Solar is an intermittent energy source, which is not what utility operators want when they need more energy immediately.